NEW U-PB ZIRCON AGES FROM THE CALEDONIAN HIGHLANDS, SOUTHERN NEW BRUNSWICK, CANADA: IMPLICATIONS FOR MAGMATIC EVOLUTION IN AVALONIA

New U-Pb (zircon) age constraints for rock units in the Caledonian Highlands of southern New Brunswick provide additional insights about the tectonic history in comparison to other parts of Avalonia. Previous dates of ca. 618 Ma and 613 ± 2 Ma from the Broad River Group have been corroborated by additional dates from felsic tuff and rhyolite of 620 ± 5 Ma and 622 ± 1.9 Ma, respectively. Combined with dates ranging from ca. 625 Ma to 615 Ma from cross-cutting plutons, the age data confirm the importance of this major episode of continental margin subduction-related igneous activity which also characterizes most other parts of Avalonia. However, a quartz-feldspar porphyry dyke yielded an igneous crystallization age of about 690 Ma, providing a minimum age for the host rocks of the dyke, mafic volcanic rocks of the Long Beach Formation which forms a faulted bounded sliver along the Bay of Fundy coast. The ages of the volcanic and less abundant sedimentary rocks of the Coldbrook Group were constrained previously by U-Pb (zircon) ages between 560 and 550 Ma as well as by similar ages from the comagmatic plutons that intruded them. Recent dating of additional samples from volcanic, plutonic, and epiclastic units all lie within this same age range. The precision of these ages is not adequate to pin down the stratigraphic details among these units but the narrow range of 560-550 Ma, including errors, demonstrates that this voluminous bimodal magmatism including extensive ash-flow tuff units formed in 10 million years or possibly much less, indicative of a "supervolcano" environment, not yet recognized elsewhere in Avalonia. A few units in the Caledonian Highlands have yielded ages that remain difficult to interpret. They include tuff of the Saddleback Brook Formation on Mount Theobald which yielded one younger age of 488 Ma (Ordovician), felsite of the Bloomsbury Mountain Formation with zircon apparently as young as 427 Ma (Silurian), and the Grassy Lake Formation with several zircon grains at 383 ± 8 Ma, suggesting a possible maximum age of mid-Devonian. In addition, preliminary LA-ICP-MS results indicate the presence of ca. 590 Ma and 565 Ma plutons in the easternmost Caledonian Highlands, an area with currently few age constraints.